Microorganisms and Plant Life 189 



Parasitism 



Mycotrophy has been interpreted by some investigators 

 as parasitism, but, if so, development of the parasite is 

 limited and is controlled by the host. Extreme parasitism, 

 resulting in diseases of plants, is a characteristic of many 

 microorganisms, including various bacteria and filamen- 

 tous fungi. Although the number of pathogens is large, 

 it is not evident why the number is not greater and why 

 there is such specificity between parasite and host. Al- 

 though the surface structures and other anatomical char- 

 acteristics of plants may provide some protection, it is 

 unreasonable to believe that these barriers are the principal 

 means whereby plants are protected from destruction by 

 microorganisms. The fact that the resistance of some varie- 

 ties of plants is correlated with anatomical differences, 

 such as epidermis composition or thickness, has limited 

 significance in explaining the general basis of plant re- 

 sistance (46). In this connection, Orton (27) said: "It is 

 hard to understand why a thick cell wall should protect 

 from infection a leaf which has many thousand openings 

 as breathing pores through which a fungus might enter 

 . . ." Furthermore, mention has already been made of 

 both intracellular and intercellular penetration of plant 

 tissues by microorganisms where the degree of penetration 

 has been controlled apparently by something other than 

 physical structures. 



It is particularly difficult to understand how plant roots 

 remain vigorous when they are coated with cells of micro- 

 organisms, including types able to decompose the various 

 substances of which the roots are composed. These root 

 structures should be ineffective barriers against penetra- 

 tion by such soil microorganisms. Certain organic sub- 

 stances such as tannin, catechol, protocatechuic acid, other 

 phenolic substances, alkaloids, organic acids, and esters may 

 serve to protect specific tissues, but it has not been shown 



